The present invention relates generally to materials for use in ophthalmic lenses and methods for making them and, more specifically, to such materials that provide for high impact resistance, low density, and good optical properties.
Ophthalmic lenses must provide good optical performance, without sacrificing other important properties, such as weight and safety. Therefore, in addition to providing visual acuity, the lenses should not be too heavy for ease and comfort of use, and they should have high resistance to breakage from impact. Generally, the materials used for making such lenses lead to tradeoffs between these desired characteristics. For example, silica glass exhibits excellent optical properties, but it is a heavy (i.e., dense) material. Glass also has poor impact resistance, resulting in the need for thick lenses, which leads to greater heaviness. Standard hard resin plastic lenses are lighter than those made of glass. However, they exhibit even worse impact resistance. An alternate material to these is polycarbonate. Polycarbonate has low density and a high refractive index, resulting in thinner lenses for a given amount of refraction. Polycarbonate also is highly impact resistant, allowing thin lenses to be used safely, without excessive risk of breakage. However, polycarbonate lenses exhibit greater chromatic aberration (i.e., blurring due to different indices of refraction for different wavelengths of light) than lenses made from glass or hard plastic resin, which may be perceived as off-axis or peripheral distortion, or color banding. Because of the limitations of these materials, improved materials have been sought for ophthalmic lenses.
A new class of impact-resistance polyurethanes has been identified that provide improved properties when used in ophthalmic lenses. U.S. Pat. Nos. 5,962,617 and 6,217,505, both to Slagel (respectively the “Slagel '617” and “Slagel '505” patents), herein incorporated by reference, describe initial formulations of these polyurethanes. These polyurethanes can be described as non-elastomeric, engineering plastics characterized by high optical quality and good impact resistance. While an elastomer will elongate at least 200% and return approximately to its original length, an engineering plastic will not undergo significant elongation. Thus, an engineering plastic retains its shape under the stresses of surfacing, grinding and polishing, which provides for superior ease of use and performance when the plastic is used for making ophthalmic lenses.
In the Slagel '617 and '505 patents, these polyurethanes are produced by reacting a polyurethane prepolymer with an aromatic amine curing agent in specific ratios to achieve the desired optical and mechanical properties. Preferred diamine curing agents for making materials for ophthalmic applications include methylenebis(ortho-chloro)aniline, 4,4′-methylenebis(3-chloro-2,6-diethylaniline), and diethyl toluene diamine. These curing agents are reacted with the prepolymer in an equivalent ratio of 0.9 to 1.1 for NH2:NCO concentration.
Though the materials described in the Slagel '617 and '505 patents did show improved impact resistance, they exhibited excessive residual yellowness that was unacceptable for use in standard ophthalmic lenses. The patents disclosed that this yellowness is addressed by addition of small amounts of colorants and also of antioxidant to protect the polyurethane from oxidation. Additionally, these base materials result in lenses that are too flexible to maintain accurate optical power when surfaced using standard optical grinding, polishing, and edging techniques. To address this excessive flexibility, the materials were modified by incorporating additional cross-linking agents to increase the materials' stiffness. For example, a preferred embodiment disclosed in the patents incorporates a small amount of trimethylol propane in the polyurethane prepolymer to improve stiffness. This is a standard technique for modifying polyurethane properties, but obviously it also increases the complexity of the system. Alternate approaches that offer simpler or more controlled routes would be desirable.
While the materials disclosed in the two patents discussed above represent a marked improvement over previously known optical materials, further refinements for ease of manufacturing and optical design implementation, lower cost of manufacture, and additional improvement of impact and other physical properties are desirable. For example, lower viscosity materials would allow for more uniform mixing of the polymer components, resulting in a more reproducible polymerization reaction. Lower viscosities also would enable production of lens and goggle shapes of greater complexity, because the material would flow more evenly and completely over the desired molding surface.
Further enhancement of physical properties also is desirable. However, such enhancement requires balancing of numerous physical, optical and chemical properties. For example, for optimal impact resistance, the material may need to flex to absorb sudden shocks. However, as discussed above, excessive flexibility results in deformation during normal surfacing and grinding procedures, so that the resulting lens may not attain or maintain the desired prescription power, or the correct shape to fit securely in the eyeglass frame. Similarly, improved resistance to environmental extremes is desirable, but not at the cost of optical clarity or unwanted color in the lens.